1 files changed, 85 insertions, 0 deletions
diff --git a/packages/lsp/src/diff.ts b/packages/lsp/src/diff.ts
new file mode 100644
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+++ b/packages/lsp/src/diff.ts
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+/**
+ * Pure diff utilities for computing LSP text document change ranges.
+ * Language-agnostic — works for any text content from any language server.
+ *
+ * Uses longest-common-prefix/suffix matching to find the minimal changed
+ * region between two versions of a file. O(n) in text length.
+ */
+
+export interface Position {
+	readonly line: number; // 0-based
+	readonly character: number; // 0-based
+}
+
+export interface TextDocumentContentChangeEvent {
+	readonly range: {
+		readonly start: Position;
+		readonly end: Position;
+	};
+	readonly text: string;
+}
+
+/**
+ * Compute the minimal change range that transforms `oldText` into `newText`.
+ * Returns a single `TextDocumentContentChangeEvent` suitable for
+ * `textDocument/didChange` with incremental sync (change: 2).
+ *
+ * Algorithm: find the longest common prefix and suffix of the two strings.
+ * The region between them is the changed range. The replacement text is
+ * the portion of `newText` between the prefix and suffix.
+ */
+export function computeChangeRange(
+	oldText: string,
+	newText: string,
+): TextDocumentContentChangeEvent {
+	const minLen = Math.min(oldText.length, newText.length);
+
+	// Longest common prefix
+	let prefixLen = 0;
+	while (prefixLen < minLen && oldText[prefixLen] === newText[prefixLen]) {
+		prefixLen++;
+	}
+
+	// Longest common suffix (must not overlap with prefix)
+	const oldRemaining = oldText.length - prefixLen;
+	const newRemaining = newText.length - prefixLen;
+	const maxSuffix = Math.min(oldRemaining, newRemaining);
+	let suffixLen = 0;
+	while (
+		suffixLen < maxSuffix &&
+		oldText[oldText.length - 1 - suffixLen] === newText[newText.length - 1 - suffixLen]
+	) {
+		suffixLen++;
+	}
+
+	const startOffset = prefixLen;
+	const endOffset = oldText.length - suffixLen;
+	const replacementText = newText.slice(prefixLen, newText.length - suffixLen);
+
+	return {
+		range: {
+			start: offsetToPosition(oldText, startOffset),
+			end: offsetToPosition(oldText, endOffset),
+		},
+		text: replacementText,
+	};
+}
+
+/**
+ * Convert a 0-based character offset into a text string to an LSP Position
+ * (0-based line and character). Scans for newlines up to the offset.
+ */
+export function offsetToPosition(text: string, offset: number): Position {
+	let line = 0;
+	let character = 0;
+	const limit = Math.min(offset, text.length);
+	for (let i = 0; i < limit; i++) {
+		if (text[i] === "\n") {
+			line++;
+			character = 0;
+		} else {
+			character++;
+		}
+	}
+	return { line, character };
+}